The present invention relates to a liquid crystal display device for displaying an image by using a liquid crystal material having spontaneous polarization and on/off driving of switching elements.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. With the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices are widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
The liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) placed on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility because the reflected light amount varies depending on environmental conditions, transmission type color liquid crystal display devices are generally used as display devices of personal computers that display full-color images.
As the color liquid crystal display devices, TN (Twisted Nematic) type color liquid crystal display devices using switching elements such as a TFT (Thin Film Transistor) are widely used. Although the TFT-driven TN type liquid crystal display devices have better display quality compared to an STN (Super Twisted Nematic) type, they require a back-light with high intensity to achieve high screen brightness because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, a lot of power is consumed by the back-light. Moreover, since a color display is achieved using color filters, a single pixel needs to be composed of three sub-pixels, and there are problems that it is difficult to provide a high-definition display, and the purity of the displayed colors is not sufficient.
In order to solve such problems, the present inventor et al. developed field-sequential type liquid crystal display devices (see, for example, T. Yoshihara, et. al., AM-LCD '99 Digest of Technical Papers, p. 185, 1999; T. Yoshihara, et. al., SID '00 Digest of Technical Papers, p. 1176, 2000). Since such a field-sequential type liquid crystal display device does not require sub-pixels, it is possible to easily realize a higher definition display compared to color-filter type liquid crystal display devices. Moreover, since a field-sequential type liquid crystal display device can use the color of light emitted by the light source as it is for display without using a color filter, the displayed color has excellent purity. Furthermore, since the light utilization efficiency is high, the field-sequential type liquid crystal display device has the advantage of low power consumption. However, in order to realize a field-sequential type liquid crystal display device, high-speed responsiveness (2 ms or less) of liquid crystal is essential.
In order to provide a field-sequential type liquid crystal display device with significant advantages as mentioned above or increase the speed of response of a color-filter type liquid crystal display device, the present inventor et al. are conducting research and development on the driving of liquid crystal such as a ferroelectric liquid crystal having spontaneous polarization, which may achieve 100 to 1000 times faster response compared to a prior art, by a switching element such as a TFT (Thin Film Transistor). In the ferroelectric liquid crystal, as shown in FIG. 1, with the application of voltage, the long-axis direction of the liquid crystal molecule is tilted. A liquid crystal panel sandwiching the ferroelectric liquid crystal therein is sandwiched by two polarization plates whose polarization axes are orthogonal to each other, and the intensity of the transmitted light is changed using the birefringence caused by the change in the long-axis direction of the liquid crystal molecule.
FIG. 2 shows an example of time chart of display control in a conventional field-sequential type liquid crystal display device. FIG. 2(a) shows the scanning timing of each line of the liquid crystal panel, and FIG. 2(b) shows the ON timing of red, green and blue colors of the back-light. One frame is divided into three sub-frames, and, for example, as shown in FIG. 2(b), red light is emitted in the first sub-frame, green light is emitted in the second sub-frame, and blue light is emitted in the third sub-frame.
Meanwhile, as shown in FIG. 2(a), for the liquid crystal panel, image data writing scanning and erasing scanning are performed within a sub-frame of each of red, green and blue colors, However, the timings are adjusted so that the start timing of writing scanning coincides with the start timing of each sub-frame, and the end timing of erasing scanning coincides with the end timing of each sub-frame, and the time required for each of the writing scanning and the erasing scanning is set to a half of each sub-frame. During the writing scanning and the erasing scanning, voltages corresponding to the same image data, which are equal in magnitude and different in polarity, are applied to the liquid crystal panel (see, for example, Japanese Patent Application Laid-Open No. 11-119189/1999).
Accordingly, when a voltage of one polarity is applied in the writing scanning, a bright display is provided, while, when a voltage of the other polarity is applied in the erasing scanning, a substantially black display is provided. More precisely, even after the erasing scanning, the same image as that displayed after the writing scanning is displayed with very low brightness compared to the image after the writing scanning. Note that in this specification, scanning for obtaining a display image with high brightness is called “writing scanning”, while, scanning for obtaining an image with low brightness, or an image perceived as a black image, is called “erasing scanning”.
A field-sequential type liquid crystal display device as described above has high light utilization efficiency and can reduce the consumption of power compared to a color-filter type liquid crystal display device. However, in the case where a ferroelectric liquid crystal is used as a liquid crystal material, since the ferroelectric liquid crystal has spontaneous polarization, if it is driven by a switching element such as a TFT, it is necessary to store charges necessary for reversing the spontaneous polarization in a liquid crystal cell through the switching element, and consequently a higher voltage is required compared to ordinary nematic liquid crystal. Moreover, a large storage capacity is necessary for storing charges. These problems also occur in a color-filter type liquid crystal display device using a ferroelectric liquid crystal, and there is a demand for a lower drive voltage. In other words, it is desired to realize a high light transmittance with a lower voltage.
On the other hand, in order to improve the display quality, there is an increasing demand for the realization of a greater number of display colors (multi-level grayscale display). However, since a liquid crystal display device employs an analog grayscale, when the number of grayscale levels is increased, the potential difference between grayscale levels decreases. Thus, from the point of view of the liquid crystal response and an output variation of the driver, there is a problem that it is difficult to achieve a multi-level grayscale display.